A direct digital radio frequency (RF) transmitter (TX) has several advantages compared to the digital-analog-RF transmitters. The direct digital-RF transmitter arranges the digital-analog interface close to the antenna, and thus fewer analog components are involved. Typical analog issues like In-phase (I) and Quadrature-phase (Q) signals mismatch, local oscillator leakage, and image distortion can be largely alleviated and even avoided. The direct digital-RF transmitter also enhances system flexibility through multi-mode and multi-band operation enabled by the agile digital signal processing. In addition, the direct digital-RF transmitter is digital friendly in nature, taking advantage of the increasing speed and density of digital processing as well as high-level integration. Thus, the direct digital-RF transmitters have benefits for both wireless base-station and mobile applications.
The direct digital-RF transmitter includes a switching mode power amplifier (SMPA), such as a class-D or class-S power amplifier, employing a particular power coding scheme, such as DSM (delta sigma modulation), PWM (pulse width modulation) and PPM (pulse position modulation) in addition with a reconstruction band-pass filter (BPF).
In order to meet the stringent linearity requirement of modern wireless communications system, most of the conventional SMPA-type transmitters use DSM as the power encoder. Examples of such modulators include band-pass delta-sigma modulation (BPDSM) based class-S power amplifiers. See, e.g., U.S. 2003/0210746, U.S. 2006/0188027, EP 2063536, and U.S. Pat. No. 7,825,724. The DSM is a noise shaping function with feedback loops, which can increase the in-band noise to the out-of-band spectrum. The in-band signal-to-noise ratio (SNR) can be greater than 60 dB.
Although the high in-band SNR is desired, the near band quantization noise can increase abruptly. Therefore, an extremely high quality factor (Q) for BPF is required to let the filtered RF signal meet the spectrum emission mask. Furthermore, the DSM based direct digital-RF transmitter can cause the overall power inefficiency, due to the low power coding efficiency of the power encoder.
In terms of power, the RF power amplifier (PA) consumes the most energy in the transmitter. A main advantage of this transmitter is that the SMPA is always between ON (saturated) and OFF (cut-off) operating region, achieving high peak efficiency. However, if non-constant envelope signals, which are common for 3rd generation (3G) and 4th generation (4G) cellular mobile communications systems, are encoded into the single bit digitized signals, the in-band power over the entire digitized signal power, defined as the power coding efficiency, is low, because the generation of quantization noise is inevitable and widely spread throughout the frequency domain due to the noise shaping function, which is required from the system linearity specification. Because this noise signal is also amplified by the SMPA, the unwanted noise power becomes wasteful, which causes both excessive power loss and total TX efficiency degeneration.
The low power coding efficiency comes from the noise shaping in delta sigma power coding scheme. Alternatively, some conventional coding schemes use various PWM techniques to address the power coding efficiency. For example, some new high-efficiency power coding schemes based on the PWM include RFPWM and 3-level polar PWM architecture. Because of the inherent nonlinearity of the PWM quantization, the linearity performance degrades in the encoder. Both power coding schemes are built with analog high-speed comparators, which use the higher frequency of triangular or saw-tooth waveform as the reference signal to be compared.
EP2575309 discloses a pre-emphasis linearization block for a 3-level PWM power coding scheme. The pre-emphasis block uses an inverse function of the transfer function of the RFPWM power coding. The output of the pre-emphasis block is submitted to the input of the RFPWM encoder. Ideally, the pre-emphasis can correct the nonlinearity by the RFPWM encoder. However, this is possible only when the inverse function exists and can be analytically derived.
For example, the system of EP2575309 uses relatively simple 3-level PWM, so the inverse function can be determined. However, for more than 3-level, e.g., 5-level RFPWM encoding, the transfer function can become so complicated that no solution can be derived for its inverse function, which leads the difficulty to build the pre-emphasis block. Therefore, this method is not suitable for high frequency transmissions requiring complex encoding.
Hence, there is a demand for a new linearization method, particularly for the high power coding efficiency power encoder.